Lignocellulosic biomass is viewed as an abundant renewable resource for fuels and chemicals due to the presence of sugars in the cell walls of plants. More than 50% of the organic carbon on the earth's surface is contained in plants. This lignocellulosic biomass is comprised of hemicelluloses, cellulose and smaller portions of lignin and protein. Cellulose is a polymer comprised mostly of condensation polymerized glucose and hemicellulose is a precursor to pentose sugars, mostly xylose. These sugars can easily be converted into fuels and valuable components, provided they can be liberated from the cell walls and polymers that contain them. However, plant cell walls have evolved considerable resistance to microbial, mechanical or chemical breakdown to yield component sugars. A number of approaches to overcome this recalcitrance have been performed and the breakdown of these polymers into sugars, saccharification, has a long history. General methods are outlined schematically in FIG. 1.
The original approaches dating back to the early 19th century involve complete chemical hydrolysis using concentrated mineral acids such as hydrochloric acid, nitric, or sulfuric acid. Numerous improvements to these processes have been made earning higher sugar yields from the biomass feedstock. These higher acid concentration approaches provide higher yields of sugars, but due to economic and environmental reasons the acids must be recovered. The primary obstacle to practicing this form of saccharification has been the challenges associated with recovery of the acid (M. Galbe and G. Zacchi, A review of the production of ethanol from softwood, Appl. Microbiol. Biotechnol. 59 (2002), pp. 618-628). Recent efforts toward separating sulfuric acid and sugars using ion resin separation or hydrochloric acid and sugars via amine extraction and subsequent thermal regeneration of the acid have been described in U.S. Pat. No. 5,820,687 and WO2010026572. Both approaches are cumbersome and expensive.
Dilute acid processes have also been attempted to perform chemical Saccharification and one such example is the Scholler-Tornesch Process. However usage of dilute acid requires higher temperatures and this usually results in low yields of the desired sugars due to thermal degradation of the monsaccharides. Numerous approaches of this type have been made in the past and all have failed to meet economic hurdles. See Lim Koon Ong, Conversion of lignocellulosic biomass to fuel ethanol—A brief review, The Planter, Vol. 80, No. 941, August 2004 and Cell Wall Saccharification, Ralf Möller, Outputs from the EPOBIO project, 2006; Published by CPL Press, Tall Gables, The Sydings, Speen, Newbury, Berks RG14 1RZ, UK.
The Saccharification of the cellulose enzymatically holds promise of greater yields of sugars under milder conditions and is therefore considered by many to be more economically attractive. The recalcitrance of the raw biomass to enzymatic hydrolysis necessitates a pretreatment to enhance the susceptibility of the cellulose to hydrolytic enzymes. A number of pretreatment methods, such as described in Nathan Mosier, Charles Wyman, Bruce Dale, Richard Elander, Y. Y. Lee, Mark Holtzapple, Michael Ladisch ‘Features of promising technologies for pretreatment of lignocellulosic biomass” Bioresource Technology 96 (2005) pp. 673-686, have been developed to alter the structural and chemical composition of biomass to improve enzymatic conversion. Such methods include treatment with dilute acid steam explosion described in U.S. Pat. No. 4,461,648, hydrothermal pretreatment without the addition of chemicals described in WO 2007/009463 A2, ammonia freeze explosion described in AFEX; Holtzapple, M. T., Jun, J., Ashok, G., Patibandla, S. L., Dale, B. E., 1991, The ammonia freeze explosion (AFEX) process-a practical lignocellulose pretreatment, Applied Biochemistry and Biotechnology 28/29, pp. 59-74, and organosolve extraction described in U.S. Pat. No. 4,409,032. Despite this, pretreatment has been cited as the most expensive process in biomass-to-fuels conversion (“Methods for Pretreatment of Lignocellulosic Biomass for Efficient Hydrolysis and Biofuel Production” Ind. Eng. Chem. Res., 2009, 48(8), 3713-3729.)
One pretreatment that has been extensively explored is a high temperature, dilute-sulfuric acid (H2SO4) process, which effectively hydrolyzes the hemicellulosic portion of the biomass to soluble sugars and exposes the cellulose so that enzymatic Saccharification is successful. The parameters which can be employed to control the conditions of the pretreatment are time, temperature, and acid loading. These are often combined in a mathematical equation termed the combined severity factor. In general, the higher the acid loading employed, the lower the temperature that can be employed; this comes at a cost of acid and its recycle. Conversely, the lower the temperature, the longer the pretreatment process takes; this comes at the cost of volumetric productivity. It is desirable to lower the temperature because pentose sugars readily decompose to form furfurals and other species which represents a yield loss and these compounds are poisons to downstream fermentation. However the use of the higher concentrations of acid required to lower the pretreatment temperatures below that where furfural formation becomes facile (B. P. Lavarack, G. J. Griffin, D. Rodman “The acid hydrolysis of sugarcane bagasse hemicelluloses to product xylose, arabinose, glucose and other products.” Biomass and Bioenergy 23 (2002) pp. 367-380) once again requires the recovery of the strong acid. If dilute acid streams and higher temperatures are employed the pretreatment reaction produces increased amounts of furfural and the acid passing downstream must be neutralized resulting in inorganic salts which complicates downstream processing and requires more expensive waste water treatment systems.